Spray drift may be defined as the physical movement of airborne spray particles through the air at the time of application or soon thereafter to any off-target site. In the agricultural context, formulations with greater spray drift characteristics are typically unfavorable because less of the formulation interacts with the target site. To counteract spray drift, drift reduction agents are typically added to the spray mixtures to reduce the amount of spray drift that occurs.
There are many drift reduction technology (DRT) tank mix adjuvants (TMA) currently on the market today. They can be lumped into broad categories based on their individual chemistries. There are natural polymers such as guar gum and other polysaccharides. There are synthetic polymers such as the polyacrylamides. These both function to increase extensional or kinematic viscosity, and this increased viscosity usually results in increased particle size and reduced fines. Reducing fines generally reduces spray drift. However, the polymer systems tend to increase the average particle size and broaden the particle size distribution (PSD). Bioefficacy can be adversely affected when mean particle size becomes too wide.
There are also oil products available that can be mineral or natural based, such as esterified seed oil. These oils produce either emulsions or invert emulsions that are thought to function by preventing small drops from forming during spray generation due to the emulsion phase present.
Surfactants are typically added to many pesticidal spray solutions to increase bioefficacy. When a surfactant is added to a spray solution the surface tension and dynamic surface tensions may be reduced. It is thought that this reduction in surface tension makes it easier to generate smaller droplets. Adding a surfactant typically reduces the average particle size and more importantly generally increases the amount of fines generated during spraying. It is the finer particles that typically drift the most. Therefore, despite their other benefits, surfactants are not typically thought of as agents to reduce spray drift.